1. Field of the Invention
This invention relates to new improved coated abrasive material, to its method of manufacture, to articles manufactured therefrom and their usage. More particularly, the invention relates to coated abrasive material having at its exposed grinding surface a coating providing improved abrading characteristics.
2. Description of the Prior Art
Coated abrasive material, which has been manufactured for many years and is commonly referred to as "sandpaper," is in general made by continuously applying to a suitable, relatively wide backing member, a first coat of an adhesive composition, referred to in the art as a "making" coat, followed by the immediate application of abrasive grains. The making coat is solidified to hold the grains on the backing member, after which, in most instances, an adhesive composition referred to as a "sand sizing" coat is applied and solidified. These coats constitute the bond for the abrasive grains and may be of the same or different chemical compositions. After further cure of the adhesive bond, the coated abrasive material thus prepared is then processed into forms more suitable for use such as sheets, rolls, belts and discs.
The use of coated abrasive articles is occasioned, in general, with a relatively high initial rate of cut. However, during use, for various reasons, the rate of cut decreases and when it falls to a rate below economic usefulness the coated abrasive article is discarded. One of the primary reasons for this decrease in cutting rate, where, e.g., metal is being abraded, and attendant with it a change in surface finish, resides in the fact that a freshly exposed metal surface is extremely reactive. Such a fresh or nascent metal surface, e.g., the metal swarf, often forms a weld with the abrasive grain apparently, in some cases at least, by means of a chemical reaction. This phenomena may result, in further usage, in an extremely high shearing force on the abrasive grain which is occasioned with even higher temperatures.
The abrasive grains on a coated abrasive article initially effective in cutting are those extending furthest from the backing member because, as is obvious, these grains first come into contact with the metal being ground. When these abrasive grains form weld junctures, frictional forces exerted thereupon by the workpiece increases, and one by one the abrasive grains are either fractured or upon failure of the grain bond are wrenched free from the backing member. As numerous of these further extending abrasive grains are fractured or removed, the abrading plane is, in general, lowered. Thus, abrasive grains become effective which do not extend outwardly from the backing material as far as the initially effective abrasive grains. As the abrading plane approaches the plane of the backing member, welded abrasive grains become more and more numerous. Ultimately the abrasive surface "glazes over."
One may already have concluded from the above that an interrelationship exists between stock removal temperature and glazing. Factors which tend to raise the local instantaneous temperature at the workpiece-abrasive interface appear to promote welding and glazing. Thus, glazing of a coated abrasive material is self-accelerating in that any increase in temperature at the workpiece-abrasive interface due to friction promotes further abrasive grain-metal welds. These welds, in turn, result in higher frictional forces and temperatures at the workpiece-abrasive interface. Undesirably, the temperature may reach a point at which the metal workpiece becomes "burned" or discolored due to heat.
Glazing, which is of course known to be more of a problem in the abrading of some metals than with others, often occurs when there is yet a substantial portion of the abrasive grains remaining, i.e., not worn down. Consequently, the abrasive article may be rendered commercially useless while the backing member is still in effective condition and before the abrasive grains' usefulness has been expended. When glazing occurs in a grinding operation, a large number of abrasive belts and the like is generally necessary to accomplish a desired surface finish. Thus, in addition to the cost of the abrasive material required to do the job, extra time and effort is consumed during the abrading operation in replacing a useless glazed over abrasive belt with a new one. All of this as is obvious, is accompanied by correspondingly higher manufacturing costs.
Various solutions over the years have been devised to eliminate or at least reduce the glazing problem. In general, these solutions have approached the problem by attempting to reduce friction between the abrasive grains and the metal surface being abraded. The use of waxes, grease sticks, and various liquid lubricants has been suggested, from time to time, for use during the abrading process. These techniques are effective only to a limited degree, and the advantages are frequently offset by the attendant inconvenience, expense, and smudging of the workpiece. Water or oil floods at the point of grinding, in addition to being inconvenient and expensive, are messy. Moreover, water necessitates the use of a waterproof coated abrasive material and oil is a potential fire hazard; the use of either requires special equipment. In any event, although lubricants such as water and oil have been found to retard glazing, for the most part they do not prevent it from ultimately occurring.
Heretofore others have suggested avoiding the problems associated with the use of external "grinding aids" by incorporating a so-called "grinding aid" in the nature of an active filler in a coated abrasive product. Merely by way of example attention is directed to U.S. Pat. No. 3,058,819. Therein is disclosed incorporated in the sand size coat an organic sulfur-containing compound such as, e.g., thiourea. Such a compound in a conventional phenol-formaldehyde binder used in the manufacture of coated abrasives, according to the patentee, inhibits glazing and welding of metal to the abrasive grain.
In recently issued U.S. Pat. No. 3,541,739 there is disclosed an antiweld filler in at least the outermost layer which may be, for example, the supersize layer, which comprises in addition to the filler an adhesive binder. Although other fillers may be included in the composition, the most critical filler is disclosed to be a simple or complex metal halide, the latter being represented by, e.g., sodium fluoaluminate, i.e., cryolite, and potassium fluoborate (potassium borofluoride).
What one might term another species of grinding aid is disclosed in U.S. Pat. No. 3,256,076. The patentee therein discloses applying to the surface of a coated abrasive sheet material, as a supersize layer, a film-forming material which comprises an organic compound containing a chemically bound substituent which decomposes at the grinding temperature and which, in the presence of normal room humidity, is rapidly reactive with or corrosive to the metal being abraded. The chemically bound constituent is chlorine, bromine or divalent sulfur which on decomposition yields HCl, HBr, or H.sub.2 S. Such a reaction, according to the patentee, is indicated by a reduction in friction between the abrasive granules and the metal surface being abraded. One may conclude, it is believed, that this invention inhibits glazing by lowering the frictional forces in grinding thus lowering the grinding temperature.
One problem heretofore associated with the use of grinding aids incorporated on the surface of the coated abrasive material, especially those in the nature of active fillers, resides in making these grinding aids continually and uniformly available at the metal surface being ground over the life of the abrasive material. Active fillers are, in general, incorporated in a binder therefor and this binder-filler composition is then applied as a layer on the surface of the coated abrasive material. Not all binder materials, however, have been found suitable. Some materials, otherwise satisfactory for binding particulate materials together and to a substrate, either do not adhere satisfactorily to the coated abrasive bond, particularly at the temperatures attained during grinding, or do not have satisfactory cohesive strength at that temperature, or both. This may result in, during usage of the coated abrasive material, flaking off of the grinding aid layer. When this occurs the maximum benefit of the grinding aid is not attained and abrasive product life is improved little, if at all.
An effective binding material for active fillers, and which most unexpectedly was found to improve grinding performance per se, was discovered by Stanley J. Supkis, one of the inventors in this application. This binder is, in general, an elastomeric material which is a copolymer of a conjugated diolefin which when polymerized results in substantial residual unsaturation. These binding materials are described more fully in application Ser No. 141,666, filed May 10, 1971, which is a continuation-in-part of Application Ser. No. 47,786, filed June 19, 1970, now abandoned.
Although these elastomeric binders, e.g., carboxylated butadiene-acrylonitrile have been found most satisfactory for coated abrasive manufacture and performance, the use of coated abrasive material incorporating these binder materials, prior to this invention, has been met, in certain instances, with some objection. This is because the binder materials emit, during usage, particularly with certain applications of the coated abrasive material, noxious odors. These odors are objectionable particularly in manufacturing plants where ventilating systems are either non-existent or inadequate for handling the odor emission.